This invention relates to the art of electroplating. More specifically it relates to the art of copper plating in an aqueous alkaline substantially cyanide-free bath.
The use of cyanide salts in copper plating electrolytes has become environmentally disfavored because of ecological considerations. Accordingly, a variety of noncyanide electrolytes for various metals have heretofore been proposed for use as replacements for the well-known and conventional commercially employed cyanide counterparts. For example, U.S. Pat. No. 3,475,293 discloses the use of certain diphosphonates for electroplating divalent metal ions; U.S. Pat. Nos. 3,706,634 and 3,706,635 disclose the use of combinations of ethylene diamine tetra (methylene phosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, and aminotri (methylene phosphonic acid) as suitable complexing agents for the metal ions in the bath; U.S. Pat. No. 3,833,486 discloses the use of water soluble phosphonate chelating agents for metal ions in which the bath further contains at least one strong oxidizing agent; while U.S. Pat. No. 3,928,147 discloses the use of an organophosphorus chelating agent for pretreatment of zinc die castings prior to electroplating with electrolytes of the types disclosed in U.S. Pat. Nos. 3,475,634 and 3,706,635.
While the electrolytes and processes disclosed in the aforementioned U.S. patents have provided satisfactory electrodeposits under carefully controlled conditions, such electrolytes and processes have not received widespread commercial acceptance as a direct result of one or more problems associated with their practice. A commercially significant problem associated with such prior art electrolytes has been inadequate adhesion of the copper deposit to zinc and zinc-based alloy and steel substrates. Another such problem relates to the sensitivity of such electrolyte systems to the presence of contaminants such as cleaners, salts of nickel plating solutions, chromium plating solutions and zinc metal ions, all of which are frequently introduced into the electrolyte during conventional commercial practice. Still another problem is the hazardous nature of strong oxidizing agents employed in certain of such prior art electrolytes.
U.S. Pat. Nos. 4,600,493 and 4,762,601 teach a process and apparatus useful in the replenishment of soluble cupric ions in an electroless copper bath. A dialysis cell employs membranes which prevent the passage of metal cations of the anode of the cell while permitting the passage of contaminant anions which are thereby removed from the electroless bath. There is no plating at the cathode; the solution in the anode compartment becomes contaminated and is therefore not suitable for return to the electroless bath.
U.S. Pat. No. 3,833,486 suggests the inclusion of a strong oxidizing agent in an electrolytic cyanide-free copper bath as a means of reducing the inefficiency resulting from the presence of contaminants. This method creates difficulties in practice because the presence of the oxidizing agent causes undesired side reactions and introduces the additional complications such as monitoring and controlling an additional bath component.
In U.S. Pat. Nos. 4,462,874 and 4,469,569, (commonly assigned) processes were proposed which provide an electrolyte which is cyanide-free, thus providing an environmentally manageable system; which claims to produce an adherent copper deposit on conductive substrates including steel, brass and zinc base metals such as zinc die casts and the like; which will efficiently produce ductile, fine-grained copper deposits at thicknesses usually ranging from about 0.015 to about 5 mils (0.000015 to about 0.005 inch); which is more tolerant of the presence of reasonable concentrations of contaminants such as cleaning compounds, salts of nickel and chromium plating solutions and zinc metal ions as normally introduced into a plating bath in a commercial practice; and which is of efficient and economical operation. The disclosures of these patents are expressly incorporated herein by reference. The processes of these patents provide for purification of the plating bath by including an auxiliary, insoluble anode in the plating bath in addition to the normal soluble copper anode. Both anodes are electrolyzed from a common bus bar. Although these processes accomplished their objective of improved deposit quality, they presented new problems. It was found that, in practice, difficulties were often encountered due to the parallel use of the two types of anodes which resulted in uncontrollable variations in current flow through the two types of anodes and a reduction in the efficiency of dissolution of the soluble copper anodes. Further, this system offered no flexibility with respect to the level of current being supplied to the insoluble anode, and was thus inefficient, as it has been discovered that the level of current needed is a fraction of that needed for the normal soluble anode--work piece cathode cell.